Elastomeric copolymers of tetrafluoroethylene (TFE) and perfluoro(methyl vinyl) ether (PMVE) exhibit outstanding resistance to chemical attack and excellent thermal stability. For a polymer to exhibit good elastomeric properties, it is necessary that the individual polymer molecules be interconnected in such a way as to form a three-dimensional network. Usually, the interconnections, or crosslinks, are formed by chemical reaction involving two or more polymer molecules. However, because of their chemical inertness, polymers of TFE and PMVE are not readily crosslinked by ordinary means, and a third monomer is necessary to serve as the crosslinking site. The resulting terpolymer can then be fabricated into shaped articles such as o-rings, gaskets, flange seals, pump diaphragms, tubing and hoses. These products are especially useful where extraordinary resistance to heat and corrosive fluids is required.
The requirements for the third monomer and the crosslinking reaction are quite stringent. The monomer must copolymerize readily, without excessive chain-transfer or inhibition. The resulting polymer must be capable of being physically mixed, or compounded, with various curatives and other materials, using conventional mixing devices such as a two-roll rubber mill or internal rubber mixer. The compound thus prepared must be such that it can withstand the elevated processing temperatures encountered during mixing and subsequent forming into the desired shapes without curing, and nevertheless subsequently cure readily to the desired crosslinked structure. Most important, the resulting crosslinks must be comparable to the dipolymer in thermal, oxidative, and chemical resistance, in order to preserve the outstanding properties of the dipolymer.
Because of these stringent requirements, it has previously been thought that completely fluorinated curesite monomers are necessary for copolymers of TFE and PMVE, and perfluorovinyl ethers of complicated structure have been preferred. Among such materials are perfluoro-(2-phenoxypropyl vinyl) ether (Pattison, U.S. Pat. No. 3,467,638), perfluoro-(3-phenoxypropyl vinyl) ether (Brizzolara and Quarles, U.S. Pat. No. 3,682,872), and perfluoro-(8-cyano-5-methyl-3,6-dioxa-1-octene) (Breazeale, U.S. Pat. No. 4,281,092). The terpolymers of Pattison or of Brizzolara and Quarles have, however, been found difficult to process and give vulcanizates having undesireably high compression-set. The terpolymers of Breazeale, on the other hand, while better in both of these respects, give vulcanizates that are not as fluid resistant in basic organic fluids. Accordingly, a need exists for curesites which give improved handling and improved compression-set resistance on the one hand, and satisfactory resistance to organic bases on the other.